Footnotes: reference International Union of Pure and Applied Chemistry, IUPAC Periodic Table of the Elements, 28th November 2016.
1. The IUPAC recognizes the commonly used alternative spellings for two elements:
The element with atomic number 13 and symbol Al may be spelt as aluminium or as aluminum.
The element with atomic number 55 and symbol Cs may be spelt as caesium or as cesium.
BUT it should be noted that these elements are present in the IUPAC periodic table as caesium and aluminium.
2. The IUPAC periodic table uses standard atomic weights and provides the lower and upper bounds of the standard atomic weight for some elements. Debate still rages about whether the term atomic mass, atomic weight, relative atomic mass, relative atomic weight, or standard atomic weight should be used. Don't worry about it, at High School you can treat all these terms as meaning the same thing.
3. The exception to this is for elements that have not yet been discovered. These elements are assigned a name based on their atomic number which will have three numbers, so the symbol for these elements has three letters. Once the element has been produced, it will be officially named and given an official symbol. If you are interested in this topic, you can found out more in the September 2012 issue of AUS-e-NEWS.
4. A whole number is a counting or natural number, it does not include negative numbers, nor numbers which are fractions or decimals etc.
5. Prior to 1961 the unit amu (atomic mass unit) was used. Since then the unit u (unified atomic mass unit) or Da (dalton) have been widely used but neither of these is an SI unit (as defined by the CIPM, International Committee for Weights and Measures). The issue will be further complicated if the dalton is redefined as being 0.001/NA kg since it would no longer be a unit of atomic mass relative to carbon-12. For High School students it is probably best to avoid the problems and use relative atomic mass (atomic weight) as a quantity with no units.
6. IUPAC recommends the term lanthanoid instead of lanthanide because the "ide" suffix is used for negatively charged ions.
Lanthanoid means "like lanthanum" so, strictly speaking, lanthanum should not be included in the list of lanthanoids, but it commonly is.
Furthermore, the "rare earth elements" are made up of scandium, yttrium and the lanthanoid elements (including lanthanum).
You can find out more about lanthanoids and rare earth elements in the December 2015 issue of AUS-e-NEWS.
7. IUPAC recommends the term actinoid instead of actinide because the "ide" suffix is used for negatively charged ions.
Actinoid means "like actinium" so, strictly speaking, actinium should not be included in the list of actinoids, but it commonly is.
